//concat
function concat(arr1, arr2) {
    return [].concat(arr1, arr2)
}

//concat
function concat(arr1, arr2) {
    return arr1.concat(arr2)
}

//push.apply
function concat(arr1, arr2) {
    let ans = arr1.slice(0);
    [].push.apply(ans, arr2);
    return ans;
}